Problem lookDevs blender 2.8 mode
There's are 2 things that affect overall lighting in the scene


lights
world


The Look Dev viewport shading allows you to see what your objects would look like in predefined lighting conditions.  This is configurable to allow standard studio lighting setups which can be useful when modeling and animating.

You can override both the scene lights and the scene world to use the lighting from your scene.

In the general case, and unless you know what you are doing you should leave both boxes unchecked (as shown in the screenshot below).

Now, there is one other thing which may cause a problem.  If this is very small then the amount of ambient light will be low and you will have a dark Look Dev setup. Make sure that the world strength setting is something sensible (probably 1.0)



I hope this helps, there's a lot of learning in blender, but also a lot of fun too.